DIY rock centerpiece- glue recommendations?

[Lady Courage]

I've gathered river rocks from the stream nearby, cleaned 'em like mad so they almost sparkle () and now am stuck at the gluing part. What do I use?
I saw the gray epoxy stuff at the store, aquarium safe and $10 for 30x the amount I need and may ever need. lol The lady said she'd never seen it in a smaller container.

So. Is there another kind of epoxy I can use? I've heard of people using silicone to glue rocks together, which is a lot cheaper and more versatile. I've heard that regular silicone is fine for aquariums, as long as it doesn't have any anti-mildew additives. But is that OK for gluing river rocks into a relatively light structure? I'm terrified of it falling down and trapping or injuring my poor fishy.

Thanks for any help y'all can offer! I'm kinda to the end of my rope on this. lol

[toad]

I resealed one of my aquariums with regular silicone from Lowe's; I think I paid $5 for the tube. So, I know that it's safe, but I don't know how it would do with holding rocks together... The only way to know is to try, I guess.